x86: Add a way to call 32-bit code from 64-bit mode

The procedure to drop from 64-bit mode to 32-bit is a bit messy. Add a
function to take care of it. It requires identity-mapped pages and that
the calling code is running below 4GB.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/Makefile

@@ -12,6 +12,12 @@ extra-y	= start.o
 obj-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
 obj-y	+= interrupts.o cpu.o cpu_x86.o call64.o
 
+AFLAGS_REMOVE_call32.o := -mregparm=3 \
+	$(if $(CONFIG_EFI_STUB_64BIT),-march=i386 -m32)
+AFLAGS_call32.o := -fpic -fshort-wchar
+
+extra-y += call32.o
+
 obj-$(CONFIG_INTEL_BAYTRAIL) += baytrail/
 obj-$(CONFIG_SYS_COREBOOT) += coreboot/
 obj-$(CONFIG_EFI_APP) += efi/

arch/x86/cpu/call32.S

@@ -0,0 +1,64 @@
+/*
+ * (C) Copyright 2015 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <asm/global_data.h>
+#include <asm/msr-index.h>
+#include <asm/processor-flags.h>
+
+	/*
+	 * rdi - 32-bit code segment selector
+	 * rsi - target address
+	 * rdx - table address (0 if none)
+	 */
+.code64
+.globl cpu_call32
+cpu_call32:
+	cli
+
+	/* Save table pointer */
+	mov	%edx, %ebx
+
+	/*
+	 * Debugging option, this outputs characters to the console UART
+	 * mov	$0x3f8,%edx
+	 * mov	$'a',%al
+	 * out	%al,(%dx)
+	 */
+
+	pushf
+	push	%rdi	/* 32-bit code segment */
+	lea	compat(%rip), %rax
+	push	%rax
+	.byte	0x48	/* REX prefix to force 64-bit far return */
+	retf
+.code32
+compat:
+	/*
+	 * We are now in compatibility mode with a default operand size of
+	 * 32 bits. First disable paging.
+	 */
+	movl	%cr0, %eax
+	andl	$~X86_CR0_PG, %eax
+	movl	%eax, %cr0
+
+	/* Invalidate TLB */
+	xorl	%eax, %eax
+	movl	%eax, %cr3
+
+	/* Disable Long mode in EFER (Extended Feature Enable Register) */
+	movl	$MSR_EFER, %ecx
+	rdmsr
+	btr	$_EFER_LME, %eax
+	wrmsr
+
+	/* Set up table pointer for _x86boot_start */
+	mov	%ebx, %ecx
+
+	/* Jump to the required target */
+	pushl	%edi	/* 32-bit code segment */
+	pushl	%esi	/* 32-bit target address */
+	retf

arch/x86/include/asm/cpu.h

@@ -229,6 +229,15 @@ char *cpu_get_name(char *name);
  */
 void cpu_call64(ulong pgtable, ulong setup_base, ulong target);
 
+/**
+ * cpu_call32() - Jump to a 32-bit entry point
+ *
+ * @code_seg32:	32-bit code segment to use (GDT offset, e.g. 0x20)
+ * @target:	Pointer to the start of the 32-bit U-Boot image/entry point
+ * @table:	Pointer to start of info table to pass to U-Boot
+ */
+void cpu_call32(ulong code_seg32, ulong target, ulong table);
+
 /**
  * cpu_jump_to_64bit() - Jump to a 64-bit Linux kernel
  *